In marine seismic exploration, operators on a boat generate sound waves from a location remote from the boat. The boat tows a cable (often times referred to in the trade as "a streamer"), in which sensors are disposed to detect the reflection of the sound waves from the ocean floor. In order to obtain accurate data, geologists prefer to employ many sensors which are arranged in short arrays and which are closely spaced in the marine streamer. Analog transmission of such data has typically required hundreds of telecommunication channels in the marine streamer.
The above identified CGG patent suggests applying telemetry techniques to a land seismic system, where multiplexing techniques are employed to reduce the number of wires to a single, two wire transmission line. A plurality of geophones were connected to the transmission line, and each geophone was connected via its own multiplexing circuit to the transmission line. The multiplexing circuits for the respective goephones were connected in parallel to the transmission line. Each circuit stored an address representative of its order of connection to the line. The multiplexing of the signals described in the CGG patent required a pulse generator at the end of the transmission line opposite the recording unit which was recording the digitized seismic signals. The system described in the CGG patent suffered from both being limited to a small number of data channel in the transmission line and from having relatively slow data transmission rates.
In contrast to the parallel multiplexing arrangement of the CGG patent, the SERCEL patent disclosed a method of connecting the digitizing modules in series in the transmission line and were to be operated as follows. The acquisition of seismic data was a three-step process. First, a synchronization signal would be sent to all the modules to alert them that a request for data was coming. Then, multiple identical requests for data would be sent to all the modules. As a module received such a request, it would put its data in the time gap behind the request, and alter the request so that subsequent modules would not recognize the request, thus allowing the request to pass through with its data to the central recording unit. Also, as each module received and responded to a request for data, it would deactivate itself so as to be unresponsive to future requests for data, until it received another synchronization signal.
Although the system disclosed in the SERCEL patent allowed both the pulse generator and recording unit to be located at the shipboard end of the transmission line, it still had limitations. The modules were all identical and not individually addressable. Although having identical modules facilitated repair and replacement, a specific module could not be communicated with or controlled without communicating with and controlling the remaining modules. Thus, the Sercel patent did not appear to disclose selective addressing or commanding. Another limitation was that the Sercel patent disclosed multiplexing words containing only 18 bits of data in each 32 bit word, resulting in a reduced transmission efficiency. Another limitation was that since all the requests for data returned to the boat in an altered form, there was no way to see if the request had been properly transmitted.
The WESTERN '553 patent attempted to solve the problem of non-addressable modules, while retaining replacement and repair advantages realized by utilizing identical modules. It disclosed a method of propagation delay addressing. In such a method, an interrogation signal and a control signal are placed on respective transmission lines having differing propagation velocities. By controlling when the interrogation and control signals are generated on the respective transmission lines, the module at which the control and interrogations signals become coincident is thus "addressed." Although the WESTERN '553 patent disclosed identical addressable modules, it still had significant limitations. Standard use required the modules to be evenly spaced along the cable, and three different transmission links were required: one each for interrogation, control, and data, although the patent did theorize that the interrogation and control channels could be multiplexed onto a single line.
The WESTERN #923 patent appears to be an improvement over the previous patent, WESTERN #553, involving in part the determination of the location of a digitizing module based on the time interval between the application of an interrogation signal and a time of receipt at the control center of the digital signal from the digitizing module.
The WESTERN #629 patent appears to apply the above principles of the Western '553 patent to a marine streamer that is towed behind a boat. However, the system described in these patents is not the optimum solution to marine streamer needs.
Generally, the systems of the WESTERN patents appear to require three links (two twisted pairs and one coaxial cable) in order to receive data (in the form of digitized data words) from the modules. A further limiting factor is the stated need for a wide bandwidth of 100 MHz. (This need is stated on page 14 under "Remarks" in one of the papers in the file history of the WESTERN #629 patent.)
It is believed desirable to have a more versatile module, which will require only a single twisted-pair transmission link for data transmission to the central unit. Such a module optimally would not require a wide bandwidth and short data words, could be selectively addressed, would be responsive to a wide variety of commands, and would have a high transmission efficiency.
The MOORCROFT patent limits its disclosure of addressing modules to simultaneous sampling at each array module (Column 2, lines 42-45; Column 3, lines 48-51; Column 5, lines 55-56; and Column 6, lines 1-3 and 28-31), implying that all modules are addressed together as a group. Furthermore, the patent discloses no means for uniquely addressing one single module to the exclusion of all other modules in the system.